1. Field of the Invention
The invention relates to an internal gear pump which may be constructed both as ring gear pump and as filling piece pump.
2. Description of the Prior Art
Such internal gear pumps must pass through a very wide speed range. They should have good volumetric efficiency at low speed and must therefore be made with narrow leak gaps. At the same time, however, at high speeds they should, as far as possible, not cause any cavitation noises due to vapour and air bubble cavitation on passage of the pumping medium from the suction side to the pressure side of the pump. These gear pumps are preferably employed as lubricating, delivery and shift or control pumps in internal-combustion engines and automatic transmissions in which in particular cavitation noises are found to be very annoying.
As a rule these gear pumps have a critical speed of rotation above which the delivery line deviates from the linear path and becomes increasingly flatter. The diagram according to the attached FIG. 1 shows the delivery stream QH (ordinate) as a function of the speed n (abscissa) and the deviation of the delivery line from the linear region from a critical speed n.sub.krit. The delivery line then becomes increasingly flatter.
From the critical speed n.sub.krit onwards, the filling degree therefore becomes smaller than 1 and consequently there is a delivery medium shortage in the teeth chambers compared with the geometric delivery volume. The shortage space is partially filled with vapour of the delivery medium, partially with air separated from the medium and partially with "false air" sucked in through leakage points. This critical speed is fundamentally defined by a critical peripheral speed in the toothing region at which in accordance with Bernoulli's Law the static pressure in the liquid is increasingly absorbed by the velocity pressure (dynamic pressure). If the static pressure drops below the vapour pressure of the liquid, bubbles are formed which are subjected to the reduced static pressure and do not condense again until the static pressure of the bubble has risen above the vapour pressure.
It is remarkable that the critical speed of the gear pumps being considered here is almost independent of the viscosity of the medium. Normally, it would be expected that the critical speed would be substantially lower in the case of a very viscous medium than in the case of a thinly liquid medium. This is however not the case. A plausible explanation of this phenomenon is seen in that the dynamic pressure is linearly dependent only on the specific mass and is dependent upon the square of the velocity. Consequently, in similar pumps having about the same peripheral velocity the critical speed is also fairly exactly at the same point irrespective of the viscosity and the design of the pump (i.e. whether with or without filling piece). In practically no case has it proved possible to influence substantially the critical speed above which the pumps become appreciably louder by modifying the tooth flank forms or the inlet passage in the housing or by other constructional steps.
In a particularly simple design of such a pump the pinion has only one tooth less than the ring gear, i.e. the pump is a so-called gerotor pump in which each tooth of the pinion permanently cooperates in sealing manner with the toothing of the ring gear. In this case, fundamentally any form of toothing may be employed which is suitable for a gerotor pump and ensures adequate sealing between the teeth of the pinion and ring gear even in the pressure region of the pump. Particularly suitable for such a gerotor pump is a pure cycloid toothing in which the teeth heads and gaps of the gears have the profile of cycloids which are formed by rolling of roll circles on fixed circles extending concentrically to the respective gear axes, the teeth heads of the pinion and the teeth gaps of the ring gear each having the form of epicycloids which are formed by rolling of a first roll circle, the teeth gaps of the pinion and the teeth heads of the ring gear each having the form of hypocycloids which are formed by rolling of a second roll circle, and the sum of the circumferences of the two roll circles being equal in each case to the tooth pitch of the gears on the fixed tooth circles thereof. Examples of such toothings are described in German published specification 39 38 346.6 and German patent application P 42 00 883.2-15.
However, the difference between the teeth numbers of the pinion and ring gear may also be greater than 1. It should however not be large in order to ensure that a relatively small average teeth number is sufficient and consequently large displacement cells are retained. It is therefore preferred for the teeth number difference not to be greater than three.
If the teeth number difference is greater than one, in the region opposite the point of deepest tooth engagement usually a filling piece must be provided which fills at least the peripherally centre portion of the free space between the head circles of the two gears and thus ensures the necessary sealing there. This type of pump is distinguished by particularly good running quietness.
Such pumps are suitable for example for feeding hydraulic systems. In particular, such pumps are used however as oil or hydraulic pumps for motor vehicle engines and/or transmissions. Motor vehicle engines and transmissions are operated in a wide speed range. The speed basic values may be in the ratio of 12:1 or more.
The desired delivery of the lubricating pump of an internal-combustion engine, which in automatic transmissions must additionally perform the function of supplying pressure to the hydraulic shift elements and the converter filling for protection against cavitation, both in the case of the engine and in the case of the transmission, is proportional to the speed only in the lower third of the operating range. In the upper speed range the oil requirement increases far less than the speed of the engine. It is therefore desirable to have a drive-regulated lubricating or hydraulic pump or one having a displacement adjustable in dependence upon the speed.
The most common form of a hydraulic, oil and/or lubricating pump is the gear pump because it is simple, cheap and reliable. A disadvantage is that the theoretical delivery per revolution is constant, i.e. proportional to the speed.
Hitherto, the only practicable way of avoiding the unnecessary pump performance from a certain pump speed onwards with low losses was to control the suction. Since the flow resistances increase overproportionally with increasing liquid velocity, with a throttle in the suction conduit with increasing speed the static pressure increasingly drops in the intake opening of the gear chamber until the so-called cavitation pressure threshold is reached, i.e. until the pressure drops below the vapour pressure of the oil. The displacement cell content then consists partly of liquid oil, partly of oil vapour, and partly of inspired air and is subjected to a static pressure lying appreciably beneath the atmospheric pressure. It is a simple matter, for example by correspondingly narrow suction conduits or by an orifice or alternatively in controllable manner by a suction slide valve to define or control the flow resistances in the suction conduit in such a manner that extensive adaptation of the useful displacement of the gear pump to the requirement line of the consumption is achieved.
A disadvantage with this control is once again the cavitation which occurs. For if the cell content consisting of liquid and gas subjected to a low absolute pressure is suddenly transferred to zones of higher pressure, as is inherent in the system of such pumps, the gaseous constituents of the cell content implode so violently that undesired noises, and even worse destruction of the cell walls, are the result.
To avoid these implosions, by shortening the outlet mouth in the region of the diminishing displacement cells the cell content is given enough time by gradual compression to increase the static pressure by an adequate extent so that when a cell comes into communication with the outlet passage no implosions of gas bubbles can take place therein because due to gradual reduction of the cell volume said gas bubbles have already condensed to liquid again or have dissolved in the liquid. The diminishing displacement cells must be sealed so well with respect to each other here that the expulsion pressure through the gap between the two teeth separating two consecutive displacement cells from each other cannot propagate itself to any appreciable extent against the displacement direction. The prevention of extremely high squeeze oil pressures at low speed is ensured constructionally in that on the displacement side of the pump the cells come into communication with the displacement pressure space so that if the cell is not filled completely with liquid the displacement pressure cannot become active therein. If however the cells are already completely filled with liquid on the suction side, which is the case in the lower speed range, the higher squeeze pressure in the cell opens the check valve in the direction towards the pressure displacement space so that the displaced oil can flow into the pressure space with only a slightly increased cell pressure compared with the displacement pressure, corresponding to the opening pressure of the check valve and the flow resistance thereof.
Such a construction is known from DE-PS 3,005,657. In the latter axial bores leading to the outlet passage extend over the entire pressure half of the pump in the housing and contain check valves which are spaced from the gear chamber and which open only when the pressure of the cell lying in front of the respective bore exceeds the pressure in the outlet passage.
This pump has a correspondingly large axial extent. The spring valve used can break. Also, the inconstant connection of the displacement cells to the outlet passage is disadvantageous. Finally, the pressure distribution in the pump is disadvantageous as regards avoiding cavitation-induced implosions and the pump is loud in operation.
Considerably more advantageous is the gerotor pump known from German patent 3,933,978 in which the problem of the squeeze oil removal in the diminishing displacement cells at low speed with cavitation-free operation is solved in that in the teeth of at least one gear passages connecting displacement cells adjacent the respective tooth are provided in which check valves are located which permit a flow through the respective passage only in the displacement direction. However, this pump is also undesirably loud in operation at higher speeds.